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Abstract

Undoped self-assembled GaN quantum dots (QD) stacked in superlattices (SL) with AlN
spacer layers were submitted to thermal annealing treatments. Changes in the balance
between the quantum confinement, strain state of the stacked heterostructures and
quantum confined Stark effect lead to the observation of GaN QD excitonic recombination
above and below the bulk GaN bandgap. In Eu-implanted SL structures, the GaN QD recombination
was found to be dependent on the implantation fluence. For samples implanted with
high fluence, a broad emission band at 2.7 eV was tentatively assigned to the emission
of large blurred GaN QD present in the damage region of the implanted SL. This emission
band is absent in the SL structures implanted with lower fluence and hence lower defect
level. In both cases, high energy emission bands at approx. 3.9 eV suggest the presence
of smaller dots for which the photoluminescence intensity was seen to be constant
with increasing temperatures. Despite the fact that different deexcitation processes
occur in undoped and Eu-implanted SL structures, the excitation population mechanisms
were seen to be sample-independent. Two main absorption bands with maxima at approx.
4.1 and 4.7 to 4.9 eV are responsible for the population of the optically active centres
in the SL samples.

Introduction

Self-assembled GaN quantum dots (QD) stacked in superlattices (SL) with AlN spacer
layers are known to be important nanostructures for optoelectronic applications in
the UV/visible and infrared spectral regions [1-3]. The GaN QD excitonic recombination is usually characterized by a broad band recombination
with ca. 300 meV of full width at half maximum for samples with homogeneous dot size
distribution [3]. It is well established that the GaN QD excitonic recombination can occur at photon
energies above and below the GaN bulk bandgap [1-8]. This behaviour is driven by the combined effects of the quantum confinement (QC)
of the carriers and the quantum confined Stark effect (QCSE), which is influenced
by the strain state of the stacked heterostructures [4,8,9]. The peak position of the GaN QD excitonic recombination is also known to be very
sensitive to the dot size, shape and thermal annealing treatments [3-10]. In addition, and despite the expected thermal stability of the QD photoluminescence
(PL) intensity, non-radiative processes described by different activation energies
have been reported in undoped and intentionally doped SL structures [3,6,11-14]. Indeed, the low temperature to room temperature PL intensity ratio, I(14 K)/I(RT), exhibits a sample dependent behaviour [3,6,11-14], which needs further investigation. Therefore, it is an aim of this article to address
the issue of the excitation and de-excitation mechanisms of the emission of as-grown,
thermally annealed and Eu-implanted GaN QD embedded in AlN spacers.

As-grown, annealed and Europium implanted and annealed GaN QD/AlN SL were studied
by temperature-dependent PL and photoluminescence excitation (PLE) in order to analyse
the influence of the excitation population mechanisms on the PL efficiency of the
excitonic GaN QD recombination. The excitation paths were seen to be sample independent
while different PL emission bands were detected for the non-doped and Europium doped
SL. The effects of the implantation fluence as well as its relationship with the carrier
localization in the GaN QD will be discussed.

Experimental

The GaN QD/AlN SL structures were grown by molecular beam epitaxy (MBE) on AlN/Al2O3 pseudo substrates as described elsewhere [15]. The investigation was performed on three sets of samples consisting of 10 (#1110)
and 20 (#987 and #989) nm stacks of (0001) GaN QD with AlN interlayers of 30 (#1110)
and 13 (#987 and #989) nm. The QD height has been set around 3.0 (#1110), 3.7 (#987)
and 4.2 (#989) nm from growth deposition parameters, in accordance with previous reported
optical experiments [4] and theoretical predictions [8,9]. An AlN cap layer was furthermore grown on the SL top part (270 nm for sample #1110
and 30 nm for samples #987 and #989). The GaN QD density and diameter was estimated
to be in the 1011 cm-2 and 15 to 20 nm ranges, respectively [16]. The as-grown sample #1110 was further submitted to thermal annealing treatments
at 1200°C in flowing N2 at 1 mbar pressure and placing a piece of AlN/sapphire face-to-face with the samples
as a proximity cap to protect the surface during the high temperature treatment. The
#987 and #989 GaN QD/AlN SL were implanted with high (1 × 1014-15 ions cm-2) and low (1 × 1013 ions cm-2) fluences of Europium ions; the SL structures were further submitted to post-implantation
thermal annealing in order to achieve Eu3+ optical activation [17,18].

Steady-state PL measurements were carried out between 14 K and room temperature (RT)
using for excitation photons with energy of 3.81 and 4.7 eV corresponding to the 325
nm line of a cw He-Cd laser (excitation density less than 0.6 W cm-2) and a monochromated 1000 W Xe lamp, respectively. The spot size of the two light
sources was 1 and 5 mm in diameter, so in both cases the luminescence arises from
a large number of QDs. The used excitation energies are below the AlN bandgap (approx.
6 eV). The samples were mounted in the cold finger of a closed-cycle helium cryostat
and the sample temperature was controlled in the range from 14 K up to RT. The luminescence
was measured using a Spex 1704 monochromator (1 m, 1200 mm-1) fitted with a cooled Hamamatsu R928 photomultiplier tube. For the PLE measurements,
the emission monochromator was set at the GaN QD excitonic recombination and the excitation
wavelength was scanned up to 5.2 eV. The spectra were corrected to the lamp and optics.

X-ray reflection (XRR) was performed on a high-resolution system using a Göbel mirror
to focus the beam and CuKα1,2 radiation.

Results and discussion

Figure 1 shows the temperature-dependent PL spectra of a 10-period as-grown and thermally
annealed GaN QD/AlN SL (#1110A and #1110D, respectively). The main maxima of the GaN
QD excitonic recombination occur below (before thermal annealing) and above (after
thermal annealing) the bulk GaN bandgap (approx. 3.5 eV). This suggests that the annealing
of the SL structure at 1200°C in nitrogen promotes a change in the balance between
the QC and QCSE as seen by the high-energy shift of the GaN QD recombination [7]. Among the various effects which could be responsible for the blue shift of the PL
peak position, both interdiffusion and thermally-induced strain relaxation mechanism
should be considered to explain the competition between the QC and QCSE [7]. The large number of satellites observed by XRR (Figure 1b) indicates that both the as-grown and the annealed SL have smooth interfaces and
high-crystalline quality. The PL thermal quenching measured between 14 K and RT is
mediated by different non-radiative processes as indicated by an intensity ratio,
I(14 K)/I(RT), of 4 and 2.5, respectively, for the as-grown and annealed samples, when excited
with the He-Cd laser line. This ratio is usually a measure of the carrier localization
on the QD and it can be concluded that a higher PL thermal stability was achieved
after the thermal annealing.

Figure 1.Influence of the thermal annealing on the optical (PL and PLE) and XRR properties
of GaN QD/AlN SL structures. (a) Temperature-dependent PL spectra of the excitonic recombination for a 10-period GaN
QD/AlN SL structure before and after thermal annealing at 1200°C (full lines). Normalised
RT PLE spectra monitored at the PL band maximum for the as-grown (line + closed symbols)
and annealed (line + open symbols) samples. 14 K PL and PLE spectra of an AlN layer
(dashed lines). (b) Specular X-ray reflection for the as-grown and annealed GaN QD/AlN SL structures.
The XRR determined period thickness is shown in the graph for both samples.

On the right side of Figure 1a, the RT PLE spectra for both samples show the same excitation paths for the GaN QD
excitonic recombination. This means that independent of the annealing effects the
population mechanisms, which give rise to the GaN QD emission, are identical. A large
asymmetric broad absorption band with a shoulder at approx. 4.1 eV extends to higher
energies showing a maximum between 4.7 and 5.0 eV. The low and high energy absorption
bands were also observed by others authors [12,19] and have been assigned to the excited state absorption from the GaN QD and to the
absorption by the wetting layer [12,19,20]. As our SL systems have AlN spacer layers, we must also account for potential excitation
mechanisms via the AlN host. In particular, it is well established that oxygen-related
defects in AlN samples are optically active and give rise to absorption and emission
bands in the ultraviolet spectral region [21]. In Figure 1a, the PL and PLE spectra of an undoped AlN layer is shown for comparison. The oxygen-related
emission [21] with a maximum at approx. 3.0 eV is observed under excitation with photons of approx.
4.9 eV energy. Despite the fact that the AlN layer PL band partially overlaps with
the one of the SL structures, their spectral shapes and peak position are clearly
distinct, which means that they are obviously due to different transitions. On the
contrary, the high-energy absorption band detected on the PLE spectra monitored on
the band maxima of the GaN QD excitonic recombination overlaps with the one associated
to the oxygen defect on the AlN layer, suggesting that the GaN QD emission band could
also be fed by the defect level from AlN spacers, buffer or capping layer in the SL
structures.

Two other sets of 20 periods GaN QD/AlN SL (#987 and #989) with larger quantum dots
(average QD heights of 3.7 and 4.2 nm according to [9]) were implanted with different fluences of Eu3+ ions. The SL structures were further submitted to thermal annealing treatments between
1000 and 1200°C in order to achieve Europium optical activation. The Eu3+ emission from GaN QD as well as from AlN layers were identified previously [17,18]. The structural analysis by X-ray diffraction (XRD) of the implanted and annealed
SL structures showed that high implantation fluences (1014 and 1015 ions cm-2) lead to higher lattice damage causing an expansion of the SL structure in the [0001]
direction, while lower fluence does not change the XRD characteristics of the sample
[17]. For these samples, and besides the Eu3+ luminescence, additional broad emission bands can be identified on the high energy
side, as shown in Figure 2i, ii. Independently of the annealing temperature, a very broad emission band peaked at
approx. 2.7 eV could be observed under excitation with photons of 3.81 eV energy for
samples implanted with high fluence. The similarity of the spectral shape and peak
position of the broad band with the emission detected under the same excitation conditions
in the as-implanted sample indicates that it arises from large, 'blurred' GaN QD present
in the damaged region of the implanted SL. The PLE spectra monitored at 2.7 eV is
similar to the one shown in Figure 1a for the non-implanted SL samples suggesting that the optically active defects in
the implanted SL are excited via the same paths. This is also confirmed with wavelength-dependent
PL studies as seen in Figure 2. Exciting the samples in the wetting layer and/or oxygen-related AlN defect absorption
bands (approx. 4.7 eV) makes the 2.7-eV PL always observable. Besides the 2.7-eV emission
band, the GaN QD/AlN SL structures implanted with high fluence show an additional
emission band peaked at 3.9 eV under 4.7 eV excitation. The observation of two GaN
QD emission bands suggests the presence of a bimodal size distribution in the studied
SL. Bimodality of GaN QD population in similar SL structures was previously reported
by Adelmann et al. [16] and they found that such distribution occur at high GaN coverage and/or substrate
temperature, which is the case of the analysed SL samples.

Figure 3a shows typical temperature-dependent PL spectra of both optical emitting centres (2.7
and 3.9 eV bands) observed with excitation with photons of 4.7 eV energy. A fast decrease
of the intensity of the 2.7-eV emission is seen with increasing temperature from 14
K to RT (I(14 K)/I(RT)~4). On the contrary, the high energy emission peak at 3.9 eV is seen to have
an intensity which is nearly constant up to 200 K. A slight increase of the PL intensity
was seen for higher temperatures accompanied with a small energy shift of the peak
position, commonly observed in small GaN QDs [11]. For the SL implanted with high fluence only, the GaN QD PL band due to the larger
blurred GaN QD have strong non-radiative de-excitation processes likely to be due
to the defects generated by ion implantation.

For samples implanted with lower fluence (#989(a)--Figures 2ii and 3b), a narrower GaN QD exciton recombination could be detected, for the 3.0-eV PL when
the SL is excited either with 3.81 and 4.7 eV photons energy. Compared with the GaN
QD PL detected in the as-grown SL, the emission band is shifted to higher energy similar
to the case of undoped annealed high-structural quality SL shown in Figure 1. The absence of the large broad emission at 2.7 eV is consistent with the high-structural
quality of this sample where no lattice expansion was found as confirmed by XRD [17]. For this SL sample also, a bimodal GaN QD distribution is present as shown by the
observation of two emitting bands from GaN QD at approx. 3.0 and 3.8 eV. Figure 3b shows the temperature-dependent PL of both optical centres observed with 4.7 eV excitation.
As observed for the high fluence implanted SL, the intensity ratio I(14 K)/I(RT) of the high energy GaN QD emission is practically constant up to RT while for
the 3.0 eV PL band a ratio of 1.3 was found. The small thermal quenching of the luminescence
observed for the SL implanted with lower fluence suggests that the competing non-radiative
processes are less important as expected for the lower damaged SL structure.

Conclusions

In Eu-implanted SL structures, the GaN QD recombination was found to be dependent
on the implantation fluence. For samples implanted with high fluence, a broad emission
band at 2.7 eV was tentatively assigned to the emission occurring at large blurred
GaN QD. The temperature-dependent PL analysis in this sample evidences a fast decrease
of the luminescence, consistent with the competing non-radiative relaxation processes
expected for a large defective SL. This emission band is absent in the lower fluence
implanted SL structure, which has high structural quality. In this case, the GaN QD
PL at approx. 3.0 eV evidences a smaller thermal quenching with increasing temperatures
from 14 K to RT. Additionally, the peak position of the emission shifts to higher
energy when compared with the one of the as-grown sample. This blue shift was also
observed in undoped and annealed SL showing that a change in the balance between the
QC and QCSE occur with thermal annealing treatments.

Despite the fact that different de-excitation processes occur in as-grown, annealed
and Eu-implanted SL, the optically active centres in the GaN QD/AlN SL are excited
via the same paths: two main absorption bands with maxima at approx. 4.1 and 4.7 to
4.9 eV.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

All the authors have made substantial intellectual contributions to the presented
study. VF and BD were responsible for the growth of the analysed samples, SM, EA and
KL performed the implantation and annealing treatments and carried the experiments
and data analysis of the structural samples characterization, MP, AJN and TM carried
out the acquisition of the optical data and their interpretation. All the authors
have together discussed and interpreted the results. All the authors read and approved
the final manuscript.

Acknowledgements

Funding by FCT Portugal (Ciência 2007 and PTDC/CTM/100756/2008) and by the bilateral
collaboration program PESSOA (EGIDE/GRICES) is gratefully acknowledged. M. Peres and
S. Magalhães thank to FCT for their PhD grants SFRH/BD/45774/2008 and SFRH/BD/44635/2008,
respectively.